Fix merge queries

[ulixius9]

In case merge queries for bigquery following are the cases that needs to be handled:

• The INTO keyword after MERGE is optional
• The column references after INSERT are optional for example INSERT VALUES (actual...values)
• The subquery after USING keyword can be a CTE query (this is not specific to bigquery)

part of fix for open-metadata/OpenMetadata#7427 ref1 ref2

Closes #380

[codecov]

Codecov Report

Merging #375 (2f1bfa5) into master (3c52d3d) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #375   +/-   ##
=======================================
  Coverage   99.39%   99.39%           
=======================================
  Files          42       42           
  Lines        1975     1981    +6     
=======================================
+ Hits         1963     1969    +6     
  Misses         12       12           

Impacted Files	Coverage Δ
.../parser/sqlfluff/extractors/dml_merge_extractor.py	100.00% <100.00%> (ø)
sqllineage/core/parser/sqlparse/analyzer.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

[reata]

I'll review this code tomorrow. Thanks for your contribution.

[reata]

I don't have the full picture regarding what feature in MERGE statement is ANSI compatible and what's not. Can you help take a look? Rule of thumb, if it's ANSI compatible, move the test case to test_others.py. If not, list all the supported dialect for each test case. And add some comment on what feature makes each test case dialect specific.